1. Technical Field
The invention relates to precision instruments. More particularly, the invention relates to an embedded data acquisition and control system for a non-invasive glucose prediction instrument.
2. Description of the Prior Art
The following deficiencies are noted with regard to state of the art data acquisition and control systems for such precision applications as the non-invasive measurement of blood glucose, where data are collected and digitized and where a complex device, such as a monochromator, is used as a light source.
Stepper Motor Control is Asynchronous with A/D Capture
FIG. 1 is a block schematic diagram that illustrates a prior art stepper motor control for a monochromator and an analog-to-digital conversion process. In the existing data acquisition system, a PC host 10 controls the stepper motor 12 position by sending a command via an RS232 serial link 11 to a bipolar stepper motor controller 13. The stepper motor controller then initiates signals to drive the stepper motor.
While the stepper motor is moving from position A to position B, the PC asynchronously reads the optical signals through the analog to digital converter 14.
One of the key elements of data acquisition for predicting blood glucose is to analyze the optical signal intensity associated with its wavelength over the scanned spectrum. Because the existing system controls the stepper motor and performs A/D conversions asynchronously, it introduces errors in terms of wavelength shifts of optical signals.
Open Loop Motor Position Control
In an open loop control system, the motor position feedback is not monitored. A common design trade-off is to limit the stepping rate to ensure the motor can drive the mechanical load without missing a step. Because the speed-torque curve varies with individual motor and load conditions change with each application, generally, a conservative step rate is implemented. This limits instrument efficiency.
Motor Noise
When the stepper motor is switched by a driver, it creates transients in terms of load current drawn and inductive voltage kickback. A common practice is to apply transient suppression diodes at the motor coils to reduce the voltage spike. However, the load current change is conductivity coupled through the system. As a result, the coupled noise is measured in the order of millivolts.
The invention provides an embedded data acquisition and control system for a precision instrument, such as a non-invasive glucose prediction instrument, which incorporates a complex device, e.g. a monochromator, as a light source.
One feature of the invention provides synchronization of monochromator stepper motor position and analog-to-digital converters. To minimize the wavelength shift error due to the asynchronous condition as described above, an embedded controller is provided that controls the stepper motor driver directly. The controller synchronizes the event of reading the A/D converters with each stepper motor position. Because the stepper motor controls the wavelength of the monochromator optical output, the net result is that each A/D conversion recorded by the embedded controller is precisely tracked to a specific wavelength.
Another feature of the invention provides closed loop motor position control for enhanced system performance. In the closed loop system, a position encoder is coupled to the stepper motor shaft. The encoded position signal is processed by a digital encoder at the bipolar stepper motor drive, resulting in an actual position feedback to the embedded controller. Because the embedded controller has the position feedback, it can initiate the next step command as soon as the position feedback reaches its target. The step rate is increased by reducing the time delay that was set by a conservative value as indicated in the open loop control mode. One advantage of this method is to achieve highest speed in all load conditions without missing steps.
A further feature of the invention provides optical Isolation to minimize motor noise. Thus, optical isolation is provided at a bipolar stepper motor drive and embedded controller interface. To maximize system sensitivity, optical isolation is implemented between the embedded controller and the stepper motor drive.